Method for separating hydroxymethylthiobutyric acid

ABSTRACT

The invention concerns an improved method for separating hydroxymethylthiobutyric acid by neutralising hydroxymethylthiobutyronitrile sulphuric hydrolysate, decanting and treating each phase with an organic solvent.

The present invention relates to an improved process for the preparationof 2-hydroxy-4-methylthio-butyric acid (HMTBA) in the form of an aqueoussolution and more especially to a process for improved separation from a2-hydroxy-4-methylthiobutyronitrile (HMTBN) acid hydrolysate mixture.2-Hydroxy-4-methylthiobutyric acid, an analog of 1-methionine, is usedin animal nutrition.

Cummins discloses, in U.S. Pat. No. 3,773,927, a process in which HMTBAis produced by hydrolysis of HMTBN with hydrochloric acid underconditions such that the suspension produced comprises solid ammoniumchloride, which is removed by centrifuging. The filtrate is subsequentlyvacuum distilled to remove the water. This U.S. Pat. No. 3,773,927discloses a process for the preparation of an aqueous liquid productwith a high concentration of HMTBA acid (85 to 90% by mass). This typeof product, obtained by such a process, has a strong smell and a darkcolor and comprises ester oligomers. These characteristics are probablydue to the high temperature conditions applied to a product with a lowwater content during the final dehydration stage. The otherdisadvantages of this process are a high energy consumption during thesame stage and difficulties and losses in yields during the centrifugingor the filtration.

U.S. Pat. No. 2,745,745 discloses the separation of HMTBA by extractionwith a water-immiscible organic phase, such as diethyl ether. BritishPatent No. 915 193 discloses a process for the preparation of calciumsalts of HMTBA where the latter is extracted with an ether, such asisopropyl ether or butyl ether, having a boiling point greater than thatof ethyl ether. Water is added to form an emulsion, to which calciumcarbonate or calcium hydroxide is added to precipitate the calcium saltof HMTBA. This patent does not relate to the preparation of liquidproducts comprising essentially HMTBA.

Gielkens mentions, in U.S. Pat. No. 3,175,000, that the directextraction of HMTBA from the hydrolysate gives poor yields. In thispatent, the extraction is only used as a secondary recovery means.

U.S. Pat. No. 4,524,077 discloses the liquid-liquid extraction of HMTBAby a water-immiscible organic solvent, preferably methyl isobutylketone, and then the recovery of the HMTBA from the extract obtainedcomprising a minimum of 5% by weight of water on the basis of the massof HMTBA recovered. The product thus prepared has a lighter color, lessof a smell, a lower viscosity and a better thermal stability than theproducts prepared by the conventional processes described above.

Patent EP 0 330 527 discloses a process for the separation of HMTBAwhich consists in forming two phases by addition of aqueous ammonia tothe hydrolysis solution, each of the phases, the first organic phasecomprising the HMTBA and the second aqueous phase comprising theammonium salt, are subsequently subjected to an evaporation operation,so as to remove the water. The concentrated filtered organic phase issubsequently diluted, so as to adjust its concentration to that of thecommercial solution. The ammonium sulfate obtained from the aqueousphase is contaminated by sulfur residues, it often exhibits anunpleasant smell which makes it difficult to sell or which requiresadditional treatments in order to remove the smell therefrom. Thepresent process requires a large amount of energy during the evaporationof the water from each of the phases obtained after neutralization andan additional treatment stage for the ammonium sulfate, which is alsoexpensive. The present invention has made it possible to solve theabovementioned problems.

DESCRIPTION OF THE INVENTION

The present invention relates to a novel process for the preparation of2-hydroxy-4-methylthiobutyric acid (HMTBA) and more specifically to anovel process for the separation of HMTBA. This process consists, in afirst stage, in adding a neutralizing agent, composed of ammoniumhydroxide, to the solution from the hydrolysis with sulfuric acid of2-hydroxy-4-methylthiobutyronitrile, which results in the separation ofthe medium into two phases. These two phases are composed of an organicphase, which is subjected to a stage of release of the salts by additionof an organic solvent, and of an aqueous phase, from which residual2-hydroxy-4-methylthio-butyric acid is exhaustively extracted byaddition of an organic solvent.

The system for release of the salts from the organic phase is composedof the addition of a solvent which is not very miscible with water.Mention may be made, among these solvents, of any solvent, chemicallycompatible with the medium and exhibiting little affinity for water,which makes it possible to dissolve the HMTBA. They can be chosen fromketones, aldehydes, ethers, esters, carbonates or alcohols. They arepreferably ketones of low molecular weight, such as methyl ethyl ketoneor methyl isobutyl ketone (MIBK), or ethers, such as methyl tert-butylether or diisopropyl ether, or carbonates, such as diethyl carbonate.They are more preferably methyl ethyl ketone, methyl isobutyl ketone andethyl carbonate.

The system for exhaustively extracting the HMTBA from the aqueous phaseis composed of the addition of a solvent which is not very miscible withwater. Mention may be made, among these solvents, of any solvent,chemically compatible with the medium and exhibiting little affinity forwater, which makes it possible to dissolve the HMTBA. They can be chosenfrom ketones, aldehydes, ethers, esters, carbonates or alcohols. Theyare preferably ketones of low molecular weight, such as methyl ethylketone or methyl isobutyl ketone (MIBK), or ethers, such as methyltert-butyl ether or diisopropyl ether, or carbonates, such as diethylcarbonate. They are more preferably methyl ethyl ketone, methyl isobutylketone and ethyl carbonate.

Each of the stages, release of the salts from the organic phase andexhaustive extraction of the aqueous phase, can be carried outindependently of one another or can be carried out jointly during theimplementation of the process for the isolation of the HMTBA. It ispreferable to carry out these two stages jointly and in a continuousprocess.

In the case where it is desired to release the salts from the organicsolution of HMTBA, the amount of organic solvent which is added to theorganic medium composed of HMTBA is according to a solvent/organicsolution of HMTBA ratio by weight preferably of greater than 0.3 andmore preferably still of between 0.3 and 1. It is obvious that a personskilled in the art will adjust the amount of solvent to be used to thestructure of the process. The temperature at which the release iscarried out is compatible with the nature of the solvent used and inparticular lies below its boiling point.

In the case where it is desired to exhaustively extract the HMTBA fromthe aqueous solution, it is preferable, according to a better way ofimplementing the invention, to use an amount of organic solvent withrespect to the aqueous solution comprising the inorganic salts, inparticular ammonium sulfate, drawn up according to an amount by weightof greater than 0.05 and preferably of between 0.1 and 0.5. Thetemperature at which the exhaustive extraction is carried out iscompatible with the nature of the solvent used and in particular liesbelow its boiling point.

The following stage consists in evaporating the organic solvent(s) used,which can be alike in the two stages or different. This operationrequires less energy than the operation for the removal of water carriedout in the prior art.

After evaporation of the solvent or solvents, which are optionallyrecycled for a fresh operation, an organic phase is obtained whichcomprises little water and which comprises a greatly reduced amount ofsalts, which eliminates the filtration phase carried out previously. Thephase is subsequently adjusted to the commercial assay of 88% by weightby addition of the water needed.

After two-phase separation between the organic solvent for exhaustiveextraction and the aqueous phase comprising the salts, a solution ofsalts and predominantly of ammonium sulfate is obtained, which solutionis crystallized and comprises virtually no more organic contaminants.The crystals are of better quality and exhibit virtually more smells.According to a first means of giving added value to these salts, thesecrystals are used in particular as fertilizer or are intended for anindustrial use.

According to a second means of giving added value to these salts, theaqueous solution of salts is treated by electrodialysis in order toregenerate, on the one hand, ammonia, which is optionally recycled tothe neutralization stage, and, on the other hand, sulfuric acid, which,after concentration, is recycled to the stage of hydrolysis of theHMTBN.

According to a third means of giving added value to the salts, thesolution of salts, which are essentially composed of ammonium sulfate,is treated by the thermal route in a plant, for example a sulfuricregeneration plant, so as to recover concentrated sulfuric acid whichcan be recycled directly to hydrolysis of the HMTBN.

An industrial implementation of the combined process is carried out inthe following way (cf. FIG. 1).

The neutralized stream (stream 1) is a separated by settling.

The organic phase resulting from the separation by settling is subjectedto an operation of release of the salts by a treatment with a phaseessentially composed of organic solvent (stream 6). This treatmentconsists of a liquid-liquid contact. It is carried out in a conventionalmixer-settler or in any other liquid-liquid contactor chosen from thefollowing devices: a bank of mixer-settlers, a packed column, aperforated plate column, a rotating disk column, a centrifugalextractor, a pulsed column or any other liquid-liquid contactor.

The extract and the raffinate are separated (stream 4 and stream 12).The raffinate is recycled to the stage of exhaustive extraction of HMTBAfrom the aqueous phase resulting from the separation by settling. Theextract (stream 4) is subjected to a treatment targeted at removing thesolvent. The removal is carried out in particular by evaporation,distillation or steam distillation. This removal operation is carriedout in order to obtain an HMTBA at the bottom (stream 8) which is highlydepleted in residual solvent and which is sufficiently concentrated,before an optional adjustment to the assay. The top stream (stream 5)originating from this removal of solvent is recycled

partly to the stage of release with respect to the organic phaseresulting from the separation by settling (stream 6)

partly to the stage of exhaustive extraction with respect to the aqueousphase resulting from the separation by settling (stream 7).

The aqueous phase which has been separated by settling (stream 10) issubjected to a treatment for exhaustive extraction of the HMTBA bycontact with a phase essentially composed of organic solvent (stream 7).This treatment is also carried out in a liquid-liquid contactor, whichcan be a bank of mixer-settlers, a packed column, a perforated platecolumn, a rotating disk column, a centrifugal extractor, a pulsed columnor any other liquid-liquid contactor.

The extract and the raffinate are separated (stream 11 and stream 13).The extract (stream 11) is recycled to the stage of release with respectto the organic phase resulting from the separation by settling, whereasthe raffinate (stream 13) is subjected to a treatment targeted atremoving the solvent. This treatment is in particular a distillation, anevaporation or a stripping. It is preferably carried out during acrystallization of the ammonium sulfate by evaporation.

The present invention will be more fully described with the help of thefollowing examples, which must not be regarded as limiting theinvention.

1—EXAMPLES OF Liquid-liquid Extraction which is Carried out on theAqueous Phase after Separation by Settling of the Neutralized HydrolysisStream

The object of this operation is to extract the residual HMTBA present inthe aqueous phase after separation by settling before crystallization ofthe ammonium sulfate.

The composition of the aqueous phase to be extracted is as follows:

[HMTBA] =  3.7% W/W [WATER] = 40.9% W/W [salts] = 55.4% W/W

Example of Exhaustive Extraction of this Aqueous Phase with MIBK

505.9 grams of this aqueous phase stream were brought into contact in astirred 1 liter reactor with 56.2 grams of MIBK at a temperature of 75°C. The extraction solvent/solution to be separated ratio was 11.1% W/W.After separation of the two phases by settling at 75° C., the two phaseswere withdrawn and analyzed:

The upper phase, in which the HMTBA is partially extracted, represented67.9 grams and had a load of HMTBA of 20.5% W/W. The content of salts inthis upper phase was below the detection limit of the analytical tool(potentiometry).

Thus, 74.3% of the participating HMTBA could be extracted with MIBK in asingle stage with a participating organic solvent/solution to beextracted ratio of 11.1%.

The lower phase, for its part, had a load of HMTBA of 0.97% W/W.

The partition coefficient, expressed as follows [HMTBA] upperphase/[HMTBA] lower phase, is 21.1.

2—Example of Exhaustive Extraction of the Aqueous Phase with EthylCarbonate

505.9 grams of the aqueous phase were brought into contact in a 1 literstirred reactor with 56.2 grams of ethyl carbonate at a temperature of75° C. The extraction solvent/solution to be separated ratio was 11.1%W/W. After separating the two phases by settling at 75° C., the twophases were withdrawn and analyzed:

The upper phase, in which the HMTBA is partially extracted, represented67.3 grams and had a load of HMTBA of 14.2% W/W. The content of salts inthis upper phase was below the detection limit of the analytical tool(potentiometry).

Thus, 50.3% of the participating HMTBA could be extracted with ethylcarbonate in a single stage with a participating organicsolvent/solution to be extracted ratio of 11.1%.

The lower phase, for its part, had a load of HMTBA of 1.53% W/W.

The partition coefficient, expressed as follows [HMTBA] upperphase/[HMTBA] lower phase, is 9.3.

3—Example of Exhaustive Extraction of the Aqueous Phase with MethylEthyl Ketone

505.9 grams of aqueous phase were brought into contact in a stirred 1liter reactor with 56.2 grams of MEK at a temperature of 75° C. Theextraction solvent/solution to be separated ratio was 11.1% W/W. Afterseparating the two phases by settling at 75° C., the two phases werewithdrawn and analyzed:

The upper phase, in which the HMTBA is partially extracted, represented65.3 grams and had a load of HMTBA of 23.7% W/W. The content of salts inthis upper phase was below the detection limit of the analytical tool(potentiometry).

Thus, 82.9% of the participating HMTBA could be extracted with MEK in asingle stage with a participating organic solvent/solution to beextracted ratio of 11.1%.

The lower phase, for its part, had a load of HMTBA of 0.6% W/W.

The partition coefficient, expressed as follows [HMTBA] upperphase/[HMTBA] lower phase, is 39.5.

4—Examples of Liquid-liquid Release Carried out on the Organic Phaseafter Separation by Settling of the Neutralized Hydrolysis Stream

The object of this operation is to purify the organic phase from salts(NH4)2SO4 and NH4HSO4) by displacement of these salts into an aqueousphase which is formed by addition of a solvent exhibiting littleaffinity for water.

The composition of the organic phase resulting from the separation bysettling of the neutralized hydrolysis stream is as follows:

[HMTBA] = 66.6% W/W [WATER] = 22.4% W/W [salts] =   11% W/W

Example of Extraction of the Salts by Addition of MIBK

379 grams of this organic phase stream were brought into contact in a 1liter stirred reactor with 137.3 grams of MIBK at a temperature of 75°C. The extraction solvent/solution to be separated ratio was 36.2% W/W.After separation by settling at 75° C., the two phases formed werewithdrawn and analyzed:

The upper phase, in which the HMTBA was extracted, represented 445.7 gand had a load of salts of 1%. Thus, with the participating organicsolvent/solution to be treated ratio used, 89% of the salts present inthe medium before extraction could be displaced into the aqueous phasecreated by the addition of MIBK.

The lower phase, into which the salts are partially displaced,represented 70.2 grams and had a load of HMTBA of 2% W/W. Thus, underthese displacement conditions, 99.4% of the participating HMTBA could beextracted with MIBK in one stage with a participating organicsolvent/solution to be treated ratio of 36.2%.

The lower phase, for its part, had a load of salts of 53% W/W.

The partition coefficient, expressed as follows [salts] lowerphase/[salts] upper phase, is 53.

5—Example of Extraction of the Salts by Addition of Ethyl Carbonate

379 grams of the organic phase stream were brought into contact in astirred 1 liter reactor with 137.3 grams of ethyl carbonate at atemperature of 75° C. The extraction solvent/solution to be separatedratio was 36.2% W/W. After separating by settling at 75° C., the twophases formed were withdrawn and analyzed:

The upper phase, in which the HMTBA was extracted, represented 455.5 gand had a load of salts of 2.4% W/W. Thus, with the participatingorganic solvent/solution to be treated ratio used, 73.9% of the saltspresent in the medium before extraction could be displaced into theaqueous phase created by the addition of ethyl carbonate.

The lower phase, into which the salts are partially displaced,represented 60.8 grams and had a load of HMTBA of 2.5% W/W. Thus, underthese displacement conditions, 99.4% of the participating HMTBA could beextracted with ethyl carbonate in one stage with a participating organicsolvent/solution to be treated ratio of 36.2% W/W.

The lower phase, for its part, had a load of salts of 50.6% W/W. Thepartition coefficient, expressed as follows [salts] lower phase/[salts]upper phase, is 21.1.

6—Example of Extraction of the Salts by Addition of MEK

379 grams of the organic phase stream were brought into contact in astirred 1 liter reactor with 137.3 grams of MEK at a temperature of 75°C. The extraction solvent/solution to be separated ratio was 36.2% W/W.After separating by settling at 75° C., the two phases formed werewithdrawn and analyzed:

The upper phase, in which the HMTBA was extracted, represented 467 g andhad a load of salts of 2.1% W/W. Thus, with the participating organicsolvent/solution to be treated ratio under consideration, 76.5% of thesalts present in the medium before extraction could be displaced intothe aqueous phase created by the addition of MEK.

The lower phase, into which the salts are partially displaced,represented 49.3 grams and had a load of HMTBA of 2% W/W. Thus, underthese displacement conditions, 99.5% of the participating HMTBA could beextracted with MEK in one stage with a participating organicsolvent/solution to be treated ratio of 36.2% W/W.

The lower phase, for its part, had a load of salts of 64.8% W/W.

The partition coefficient, expressed as follows [salts] lowerphase/[salts] upper phase, is 30.9.

What is claimed is:
 1. A method for separating2-hydroxy-4-methylthiobutyric acid (HMTBA) from a2-hydroxy-4-methylthiobutyronitrile (HMTBN) acid hydrolysate mixturecomprising HMTBA and water, comprising the following steps: neutralizingthe mixture with aqueous ammonia wherein the amount of water added tothe hydrolysate mixture is sufficient to cause the formation of aseparate aqueous phase; settling the neutralized mixture to obtain anorganic phase and an aqueous phase; separating the organic phase, whichcomprises HMTBA, water and salts, from the aqueous phase, whichcomprises HMTBA, water and salts; releasing the salts from the organicphase with an amount of a first organic solvent which is not verymiscible with water by contacting said organic phase with said firstorganic solvent; exhaustively extracting the HMTBA from the aqueousphase with an amount of a second organic solvent which is not verymiscible with water by contacting said aqueous phase with said secondorganic solvent; and evaporating said first and second organic solvents,wherein said first and second organic solvents are the same ordifferent, further wherein most of the salts are precipitated from theaqueous phase after the aqueous phase has been separated from theorganic phase.
 2. A method for separating 2-hydroxy-4-methylthiobutyricacid (HMTBA) from a 2-hydroxy-4-methylthiobutyronitrile (HMTBN) acidhydrolysate mixture comprising HMTBA and water, comprising the followingsteps: neutralizing the mixture with aqueous ammonia wherein the amountof water added to the hydrolysate mixture is sufficient to cause theformation of a separate aqueous phase; settling the neutralized mixtureto obtain an organic phase and an aqueous phase; separating theneutralized mixture into two phases wherein the first phase is theorganic phase comprising HMTBA, water and salts and the second phase isthe aqueous phase comprising HMTBA, water and salts; treating the firstphase, separately from the second phase, with a first organic solvent torelease the salts from the first phase, wherein said first organicsolvent is not very miscible with water; treating the second phase,separately from the first phase, with a second organic solvent toexhaustively extract the HMTBA from the second phase, wherein the secondorganic solvent is not very miscible with water; and evaporating saidfirst and second organic solvents, wherein said first and second organicsolvents are the same or different, further wherein most of the saltsare precipitated from the aqueous phase after the aqueous phase has beenseparated from the organic phase.
 3. The method of claim 1, wherein theorganic phase is contacted with the first organic solvent in a firstcontainer and the aqueous phase is contacted with the second organicsolvent in a second container that is separate from said firstcontainer.
 4. The method of claim 2, wherein said first organic solventis selected from the group consisting of ketones, aldehydes, ethers,esters, carbonates and alcohols, provided they dissolve the HMTBA. 5.The method of claim 2, wherein said second organic solvent is selectedfrom the group consisting of ketones, aldehydes, ethers, esters,carbonates and alcohols, provided they dissolve the HMTBA.
 6. The methodof claim 1, wherein the first organic solvent and the second organicsolvent are identical.
 7. The method of claim 1, wherein the firstorganic solvent and the second organic solvent are different.
 8. Themethod of claim 1, wherein said first organic solvent is selected fromthe group consisting of ketones, aldehydes, ethers, esters, carbonatesand alcohols, provided they dissolve the HMTBA.
 9. The method of claim8, wherein said first organic solvent is selected from the groupconsisting of ketones of low molecular weight, ethers and carbonates.10. The method of claim 9, wherein the first organic solvent is selectedfrom the group consisting of methyl ethyl ketone, methyl isobutylketone, methyl tert-butyl ether, diisopropyl ether and diethylcarbonate.
 11. The method of claim 1, wherein the amount of the firstorganic solvent used with respect to the organic phase is greater than0.3 (w/w).
 12. The method of claim 11, wherein the amount of the firstorganic solvent used with respect to the organic phase is between 0.3and 1 (w/w).
 13. The method of claim 1, wherein the second organicsolvent is selected from the group consisting of ketones, aldehydes,ethers, esters, carbonates and alcohols, provided they dissolve theHMTBA.
 14. The method of claim 13, wherein the second organic solvent isselected from the group consisting of ketones of low molecular weight,ethers and carbonates.
 15. The method of claim 14, wherein the secondorganic solvent is selected from the group consisting of methyl ethylketone, methyl isobutyl ketone, methyl tert-butyl ether, diisopropylether and diethyl carbonate.
 16. The method of claim 1, wherein theamount of the second organic solvent with respect to the aqueous phaseis greater than 0.05 (w/w).
 17. The method of claim 16, wherein theamount of the second organic solvent with respect to the aqueous phaseis between 0.1 and 0.5 (w/w).
 18. The method of claim 1, wherein theexhaustive extraction of HMTBA and the release of the salts are carriedout concomitantly and according to a continuous process.
 19. The methodof claim 1, wherein after the salts have been released from the organicphase, at least a portion of the first organic solvent is separated fromthe HMTBA and recycled so that it becomes at least a portion of thesecond organic solvent.
 20. The method of claim 1, wherein the saltsobtained from the aqueous phase are crystallized.
 21. The method ofclaim 20, wherein the resulting salt crystals are treated byelectrodialysis in order to generate ammonia, which is recycled tobecome part of the aqueous ammonia, and sulfuric acid, which, afterconcentration, is recycled for use in obtaining the HMTBN acidhydrolysate mixture.
 22. The method of claim 20, wherein the resultingsalt crystals are thermally treated to recover a sulfuric acid solutionwhich is recycled for use in obtaining the HMTBN acid hydrolysatemixture.
 23. A method of using the salts recovered from the aqueousphase in the process of claim 20, comprising the step of applying thesalts to soil or plants as fertilizer.